The present invention relates to a protection circuit, integrated circuit and host device for the protection of batteries.
At present, it is very important for the users of different electronic devices that the electronic device can be used as long as possible before it is necessary to charge the battery. Furthermore, especially in portable devices, the size of the battery is significant, and thus it is not necessarily reasonable to reduce the need to charge the battery by increasing the capacity of the battery. Therefore, especially in wireless communication devices and in portable computers, the use of Lithium-based batteries, such as Li-ion (Lithium ion), Li-poly (Lithium polymer) or Li-metal (Lithium metal) batteries has become increasingly common.
A Li-ion battery is considerably lighter and has a somewhat larger capacity than NiCd and NiMH batteries, and thus considerably longer operating times are attained without increasing the size of the battery. On the other hand, the manufacture of a Li-ion battery is far more expensive than the manufacture of NiCD and NiMH batteries. Recharging of a Li-ion battery does not require that the battery is (fully) discharged. On the other hand, the longest possible service life is obtained from NiCD batteries if the battery is discharged completely before recharging. In Li-ion batteries self-discharging is less than e.g. in NiCD batteries (approximately 1 to 2% per month), and thus an unused Li-ion battery may retain its charge for a comparatively long time. In subzero temperatures, the operation of a Li-ion battery is similar to that of NiMH batteries, in other words it is not particularly good.
An advantage of the Li-poly battery is that it is easier to manufacture and it is possible to make the battery smaller and lighter than the Li-ion battery. A Li-poly battery can be shaped quite freely. The self-discharge rate of a Li-poly battery is even smaller than that of a Li-ion battery.
Li-ion and Li-poly batteries should be protected from over-voltage and under-voltage by means of a rather complex protection circuit, because otherwise the cells of the battery can be damaged so that they become unusable. The most important rule when charging Li-ion and Li-poly batteries is to keep the charging voltage as constant as possible during the entire charging process. Normally, the charging voltage is either approximately 4.1 V or approximately 4.2 V. The purpose of the protection circuit is to interrupt the charging process when a particular voltage is attained, for example 0.15 V over the charging voltage. After the operation of the over-voltage protection circuit, the battery can nevertheless be discharged. When the battery has been discharged, it can be charged again. In addition to too high a voltage (over-voltage), Li-ion and Li-poly batteries are particularly sensitive to too low a voltage (under-voltage) and to over-current when they are charged or discharged. In these cases, the purpose of the protection circuit is to interrupt the discharging or charging of the battery.
In order to implement the functionality of the protection circuit, the protection circuit should advantageously contain at least a control block and two switch means such as two field-effect transistors (FET), connected in series. One field-effect transistor protects the battery from over-voltage and the other from under-voltage. By means of this arrangement of two field-effect transistors it is possible to enable the battery to be discharged after an over-voltage condition and to be charged after an under-voltage condition.
Because of parasitic diodes internal to the field-effect transistor, current can be passed in the opposite direction through the field-effect transistor from the drain to the source when the field-effect transistor is in a high impedance state. This enables a battery protected by the protection circuit to be discharged after an over-voltage condition and to be recharged after an under-voltage condition.
In a particular prior art solution, a low impedance resistance is connected in series in the voltage supply line of the battery. The voltage across this resistance is measured, wherein an over-current condition can be detected when the voltage exceeds a predetermined limit. The use of components that increase the impedance is not desirable, because they reduce the voltage supplied to the electronic device and unnecessarily increase power consumption. Thus, the operating time of the device using the battery is shortened.
In another prior art solution, an over-current condition is detected in such a way that the voltage across the drain and source of the field-effect transistor is measured. Additionally, the value of the resistance between the drain and source, the so-called conducting state drain-to-source resistance Rds(on), is estimated. In prior art solutions, this drain-to-source resistance is presumed constant. Thus, an estimate of the current is obtained by dividing the voltage across the drain and the source of the field-effect transistor by the drain-to-source resistance. One disadvantage of this solution is that the drain-to-source resistance is not constant, but changes as the gate voltage of the field-effect transistor changes. Moreover, the drain-to-source resistance depends to a considerable degree on the temperature of the field-effect transistor.
In prior art solutions over-current conditions that occur during charging are not monitored, but the battery is protected only e.g. by a fuse. Charging currents are usually smaller and easier to predict than currents that occur when the battery is being discharged, and consequently, over-current during charging has not been considered a problem. However, it is not impossible that an over-current condition can also arise during charging, for example due to a defective charging device. Thus, it is also advantageous to protect the battery from over-current during charging.
Patent application JP 10223260 discloses a protection circuit for a battery, in which the aim is to compensate the effect of temperature when measuring the current, so that more reliable measurement results are obtained. The protection circuit of the invention according to JP 10223260 comprises an over-voltage and under-voltage detection unit 2 (FIG. 1), a charging control block 3, an over-current protection block 4, a discharging-side overheating protection block 5, a charging-side overheating protection block 6 and two field-effect transistors FET1, FET2.
The purpose of the over- and under-voltage detection unit 2 is to detect when the voltage of the cells 1a, 1b, 1b of the battery is too high or too low. When a load (not shown), for example an electronic device, is connected across connectors P1, P2, in other words the battery is discharged, the over-voltage or under-voltage detection unit 2 monitors each cell 1a, 1b, 1c of the battery separately to detect an under-voltage state. If the voltage of any cell is lower than a certain first threshold value, the over-voltage and under-voltage detection unit sets line P into a first logical state, which results in the first field-effect transistor FET1 becoming non-conductive, whereupon discharging of the battery is terminated.
When a charging device (not shown) is connected across connectors P1, P2, i.e. the battery is charged, the over- and under-voltage detection unit 2 monitors each cell 1a, 1b, 1c of the battery separately to detect an over-voltage state. If the voltage of any cell exceeds a certain second threshold value, the over-voltage and under-voltage detection unit sets line L into a second logical state, which results in the second field-effect transistor FET2 becoming non-conductive, whereupon charging of the battery is terminated.
The purpose of the charging control block 3 is to control the second field-effect transistor FET 2 in such a way that when line L is in the second logical state, the second field-effect transistor FET2 does not pass a charging current, i.e. the battery is not charged. Correspondingly, when line L is in the first logical state, the second field-effect transistor passes a charging current, i.e. the battery is charged.
The purpose of the over-current protection block 4 is to interrupt discharging of the battery when the current supplied to the electronic device is too high. The over-current protection block comprises two symmetrical circuits with substantially equal properties. The circuits are connected to the drain and source of the first field-effect transistor. As the current increases, the voltage difference between the drain and the source of the first field-effect transistor also increases. When this voltage difference reaches a certain value, it causes the over-current protection block to set the first field-effect transistor into a non-conductive state. Thus the current supply to the electronic device is interrupted.
Let us assume that a load (not shown), for example an electronic device, is connected between connectors P1, P2, i.e. the battery is discharged, and the cells 1a, 1b, 1c of the battery are not in an under-voltage condition. In this situation, an over-current causes the temperature of the first field-effect transistor FET1 to rise above normal. When the first field-effect transistor reaches a certain temperature, the discharging-side overheating protection block 5 switches the first field-effect transistor FET1 into a non-conductive state, whereupon discharging of the battery is terminated.
Correspondingly, let us assume that a charging device is connected between connectors P1. P2, i.e. the battery is charged, and the cells 1a, 1b, 1c of the battery are not in an over-voltage condition. In this situation, an over-current causes the temperature of the second field-effect transistor to rise above normal. When the second field-effect transistor reaches a certain temperature, the charging-side overheating protection block 6 switches the second field-effect transistor FET2 into a non-conductive state, whereupon charging of the battery is terminated.
However, this solution has the disadvantage that it does not take into account changes in the drain-to-source resistance of the field-effect transistor as the temperature changes. As mentioned earlier in this description, a change in temperature changes the drain-to-source resistance between the source and the drain. Thus, the shut-off actually takes place at different current values at different temperatures.
It is an objective of the present invention to provide a protection circuit for batteries, such as Li-ion and Li-poly batteries, which is capable of more accurately protecting a battery from over-current, over-voltage and under-voltage when the battery is charged or discharged by taking into account the dependence of the properties of a field-effect transistor on at least one physical quantity, such as temperature and/or gate voltage. Because in the solution according to the invention, the value of the current can be determined more accurately compared with present solutions, the battery""s charge can also be measured considerably more accurately when compared with present solutions. Another objective of the invention is to avoid the introduction of additional impedance into the protection circuit.
According to the invention, the first objective can be attained by using a value of drain-to-source resistance, compensated using at least one physical quantity, such as temperature and/or gate voltage, to detect over-current. The compensation takes place in such a way that information concerning the behaviour of the field-effect transistors at different temperatures and/or different gate voltages, as well as measured temperature and/or voltage values is stored in a parameter memory of the protection circuit. This information is used to obtain a value of drain-to-source resistance which is as accurate as possible, whereupon the actual value of the current can be determined more accurately than in prior art methods. Furthermore, monitoring can take place in connection with both charging and discharging of the battery. Because the temperature and/or the gate voltage of the field-effect transistor is taken into account when the current is determined, a more precise value is also obtained for the charge of the battery compared with prior art solutions. Because the current is measured in a more precise manner when compared to prior art, it is also possible to increase the operating time of the host device. According to the invention, the second objective can be attained in such a way that an over-current is detected using the drain-to-source resistances and/or drain-to-source voltage of the field-effect transistors, whereupon additional resistors are not necessary. The protection circuit according to the invention can be advantageously implemented in an application specific integrated circuit (ASIC), wherein the battery protection circuit becomes considerably smaller and less expensive when compared to circuits where separate components are used.
In one aspect, the present invention is directed to a protection circuit. In one embodiment the protection circuit comprises at least one switch (FET1, FET2) comprising at least one control means (G1, G2) for adjusting the conductivity of at least one switch (FET1, FET2), the conductivity being arranged to be adjustable by means of an electrical control applied to the control means (G1, G2). The protection circuit (30) include. means (22, 25, 26) for forming the electrical control, means (27, 28) for measuring at least one physical quantity affecting the at least one switch (FET1, FET2), means (10) for providing information about the dependence of the conductivity properties of said at least one switch (FET1, FET2) on the at least one physical quantity, means (29) for determining the conductivity of the at least one switch (FET1, FET2) on the basis of the at least one physical quantity and the conductivity properties of the at least one switch (FET1, FET2) and means (29, 27) for determining the current (ITOT) passing through the at least one switch (FET1, FET2) at least partly on the basis of the conductivity, wherein the electrical control is arranged to be formed at least partly on the basis of the determined current.
In another aspect, the present invention also relates to an integrated circuit. In one embodiment the integrated circuit comprises a protection circuit which comprises at least one switch (FET1, FET2) comprising at least one control means (G1, G2) for adjusting the conductivity of the at least one switch (FET, FET2), the conductivity being arranged to be adjustable by means of an electrical control conducted to the control means (G1, G2). The protection circuit (30) includes means (22, 25, 26) for forming the electrical control, means (27, 28) for measuring at least one physical quantity affecting the at least one switch (FET1, FET2), means (10) for providing information about the dependence of the conductivity properties of the at least one switch (FET1, FET2) on the at least one physical quantity, means (29) for determining the conductivity of the at least one switch (FET1, FET2) on the basis of the at least one physical quantity and the conductivity properties of the at least one switch (FET1, FET2) and means (29, 27) for determining the current (ITOT) pausing through the at least one switch (FET1, FET2) at least partly on the basis of the conductivity, wherein the electrical control is arranged to be formed at least partly on the basis of the determined current.
In another aspect, the present invention also relates to an integrated circut. In one embodiment the integrated circuit comprises a protection circuit which comprises at least one switch (FET1, FET2) comprising at least one control means (G1, G2) for adjusting the conductivity of the at least one switch (FET1, FET2), the conductivity being arranged to be adjustable by means of an electrical control conducted to the control means (G1, G2). The protection circuit (30) includes means (22, 25, 26) for forming the electrical control, means (27, 28) for measuring at least one physical quantity affecting the at least one switch (FET1, FET2), means (10) for providing informatio about the dependence of the conductivity properties of the at least one switch (FET1, FET2) on the at least one physical quantityl, means (29) for determining the conductivity of the at least one switch (FET1, FET2) on the basis of the at least one physical quantity and the conductivity properties of the at least one switch (FET1, FET2) and means (29, 27) for determining the current (ITOT) passing through the at least one switch (FET1, FET2) at least partly on the basis of the conductivity, wherein the electrical control is arranged to be formed at least partly on the basis of the determined current.
In a further aspect, the invention relates to a host device. In one embodiment the host device 33, is provided with a protection circuit 30 which comprises at least one switch (FET1, FET2) comprising at least one control means (G1, G2) for adjusting the conductivity of the at least one switch (FET1, FET2), the conductivity being arranged to be adjustable by means of an electrical control conducted to the control means (G1, G2). The protection circuit (30) includes means (22, 25, 26) for forming the electrical control, means (27, 28) for measuring at least one physical quantity affecting the at least one switch (FET1, FET2), means (10) for providing information about the dependency of the conductivity properties of the at least one switch (FET1, FET2) on the at least one physical quantity, means (29) for determining the conductivity of the at least one switch (FET1, FET2) on the basis of the at least one physical quantity and the conductivity properties of the at least one switch (FET1, FET2) and means (29, 27) for determining the current (ITOT) passing through the at least one switch (FET1, FET2) at least partly on the basis of the conductivity, wherein the electrical control is arranged to be formed at least partly on the basis of the determined current.
In another aspect, the present invention relates to a battery. In one embodiment the battery 31 includes a protection circuit 30 which comprises at least one switch (FET1, FET2) comprising at least one control means (G1, G2) for adjusting the conductivity of the at least one switch (FET1, FET2), the conductivity being arranged to be adjustable by means of an electrical control conducted to the control means (G1, G2). The protection circuit (30) includes mean, (22, 25, 26) for forming the electrical control, means (27, 28) for measuring at least one physical quantity affecting the at least one switch (FET1, FET2), means (10) for providing information about the dependence of the conductivity properties of said at least one switch (FETX, FET2) on the at least one physical quantity1 means (29) for determining the conductivity of the at least one switch (FET1, FET2) on the basis of the at least one physical quantity and the conductivity properties of the at least one switch (FET1, FET2) and means (29, 27) for determining the current (ITOT) passing through the at least one switch (FET1, FET2) at least partly on the basis of the conductivity, wherein the electrical control is arranged to be formed at least partly on the basis of the determined current.
In a further aspect, the invention relates to a method for protecting a battery. In one embodiment the method for protecting a battery using a protection circuit comprises providing the protection circuit with at least. one switch (FET1, FET2) which comprises at least one control means (G1, G2) for adjusting the conductivity of the at least one switch (FET1, FET2), by means of an electrical control conducted to the control means (G1, G2). In the method at least one physical quantity affecting the at least one switch (FET1, FET2) is measured, information about the dependence of the conductivity properties of said at least one switch on the at least one physical quantity in provided, the conductivity of the at least one switch (FET1, FET2) is determined on the basis of the at least one physical quantity and the conductivity properties of the at least one switch (FET1, FET2) and the current (ITOT) conducted through the at least one switch (FET1, FET2) is determined at least partly on the basis of the conductivity, wherein the electrical control is formed at least partly on the basis of the determined current.
By means of the present invention, considerable advantages are achieved compared with solutions according to prior art. Because the protection circuit according to the invention protects a battery from over-current considerably better than prior art solutions, the operating life of the battery is extended because, as a result of more accurate over-current protection, the probability of damage to the battery is reduced. By means of the solution according to the invention, it is possible to protect a battery from over-current both when discharging and charging the battery, and thus the battery is also protected, for example, from a faulty charging device. Because over-current protection is implemented in such a way that the protection circuit does not contain unnecessary resistive components which cause power dissipation, the operating time of the device using the battery is increased. Furthermore, the protection circuit according to the invention is less expensive and smaller compared with prior art protection circuits, because it can be implemented in a single application specific integrated circuit. Because the charge of the battery can be measured considerably more accurately using the protection circuit according to the invention than in prior art solutions, it is possible to estimate e.g. the shutdown time of the device that is being used.